Industrial usage of metal parts manufactured by the compaction and sintering of metal powder compositions is expanding rapidly into a multitude of areas. In the manufacture of such parts, metal powder compositions are typically formed from metal-based powders and other additives such as lubricants, and binders. The metal-based powders are typically iron powders that may have been optionally prealloyed with one or more alloying components.
A common technique for prealloying involves forming a homogeneous molten metal composition containing iron and one or more desired alloying components, and water atomizing the molten metal to form a homogeneous powder composition.
The metal-based powder, after any optional prealloying, is often mixed with other additives to improve the properties of the final part. For example, the metal-based powder is often admixed with at least one other alloying compound or element that is in powder form ("alloying powder"). The alloying powder permits for example, the attainment of higher strength and other mechanical properties in the final sintered part.
The alloying powders typically differ from the metal-based powders in particle size, shape and density. For example, the average particle size of the metal-based powders such as iron is typically about 70-100 microns, or more, while the average particle size of most alloying powders can be less than about 20 microns, frequently less than about 15 microns, and in some cases less than about 5 microns. However, substantially pure copper containing powder has generally not been used in such small particle sizes (e.g., 20 microns or less) because the smaller size pure copper containing powder is more expensive relative to the larger particle size copper containing powder, and there has been no other incentive to use the smaller size pure copper containing powder.
The mixture of metal-based powder and optional alloying powders are often also mixed with other additives such as lubricant to form the final metal powder composition. This metal powder composition is typically poured into a compaction die and compacted under pressure (e.g., 5 to 70 tons per square inch (tsi)), and in some circumstances at elevated temperatures, to form the compacted, or "green," part. The green part is then usually sintered to form a cohesive metallic part. The sintering operation also bums off organic materials.
One problem that occurs in forming iron-based powder compositions is that the disparity in particle size between the alloying powders and iron-based powders can lead to problems such as segregation and dusting of the finer alloying particles during transportation, storage, and use. Although the iron-based powders and alloying powders are initially admixed into a homogeneous powder, the dynamics of handling the powder mixture during storage and transfer can cause the smaller alloying powder particles to migrate through the interstices of the iron-based powder matrix. The normal forces of gravity, particularly where the alloying powder is denser than the iron-based powder, cause, the alloying powder to migrate downwardly toward the bottom of the mixture's container, resulting in a loss of homogeneity of the mixture, or segregation. On the other hand, air currents which can develop within the powder matrix as a result of handling can cause the smaller alloying powders, particularly if they are less dense than the iron-based powders, to migrate upwardly. If these buoyant forces are high enough, some of the alloying particles can, in the phenomenon known as dusting, escape the mixture entirely, resulting in a decrease in the concentration of the alloy element.
One solution to the aforementioned dusting and segregation problem described has been to use various organic binders to bind or "glue" the finer alloying powder to the coarser iron-based particles to prevent segregation and dusting for powders to be compacted at ambient temperatures. For example, U.S. Pat. No. 4,483,905 to Engstrom teaches the use of a binding agent that is broadly described as being of "a sticky or fat character" in an amount up to about 1% by weight of the powder composition. U.S. Pat. No. 4,676,831 to Engstrom discloses the use of certain tall oils as binding agents. Also, U.S. Pat. No. 4,834,800 to Semel discloses the use of certain film-forming polymeric resins that are generally insoluble in water as binding agents. Despite the advantages of binders, binders can sometimes reduce compressibilities and the mechanical properties of a part.
Another solution that has been in use since the mid 1960's is to employ "diffusion bonded iron-based particles." The diffusion bonded iron-based particles are powders of substantially pure iron that have one or more other metals such as steel producing elements diffusion bonded and partially alloyed into their outer surfaces. Such commercially available powders are Distaloy.TM. AB and Distaloy.TM. AE available from Hoeganaes Corporation located in Cinnaminson, N.J. The Distaloy AB and AE metal powders are made to conform with MPIF standard 35 FD-02 and FD-04 respectively. Thus, Distaloy AB contains about 1.5 weight percent copper, about 1.75 weight percent nickel, and about 0.5 weight percent molybdenum. Distaloy AE contains about 1.5 weight percent copper, about 4.00 weight percent nickel, and about 0.5 weight percent molybdenum.
The Distaloy AB and AE metal powders are preferably prepared by the methods disclosed in British patent specification GB 1,162,702, published Aug. 27, 1969, which is hereby incorporated by reference in its entirety. In a preferred method, the Distaloy AB and AE metal powders are prepared by blending substantially pure iron powder with copper, molybdenum, and nickel containing powder additives. The substantially pure iron powder generally contains less than 0.50 weight percent residual impurities, has a maximum particle size of nominally 250 microns, and a weight average particle size of from about 60 microns to about 75 microns. The copper and molybdenum additives are typically in oxide form (e.g., cuprous oxide and molybdenum trioxide), while the nickel powder is typically in elemental form. The copper, nickel, and molybdenum additives generally have a weight average particle size of 15 microns or less. After blending the powder additives, the resulting mixture is submitted to hydrogen annealing at temperatures which typically range from about 800.degree. C. to about 900.degree. C. The annealing first reduces the copper and molybdenum oxides to elemental form. Thereafter, the reduced copper containing powder, the reduced molybdenum powder, and nickel powder partially alloy with the iron powder, and also, to some extent, partially alloy with each other through a diffusion mechanism. Because the mixture tends to agglomerate during annealing, after cooling, the mixture is typically reformed into a powder through a disintegration step. It is also sometimes desired to submit the powder, after disintegration, to a second blending step, as the mixture tends to segregate through various mechanisms during annealing and disintegration. The diffusion bonded and partially alloyed powder thus produced may subsequently be mixed with other typical additives, such as lubricants, machining agents, and graphite. Distaloy AB and AE are thus far in the industry the highest performing grades with respect to strength and impact resistance. Despite the advantages, these powders are expensive both because of the extra processing steps that are needed to perform the diffusion bonding, and the significant capital investment that is required to provide the associated processing equipment.
It would be desirable to develop alternate methods of preparing these powder metallurgical compositions. Preferably, such methods would provide powder metallurgical compositions with comparable or improved mechanical properties to the Distaloy compositions.